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Perceptual and Physical Elements to Support Rural-Urban Transitions . . . . . . . . . . . . . . . . .9-2 9-1 C H A P T E R 9 Transition Zones Between Varying Road Designs
P ER CE PT UA L AN D P HY SI CA L E LE ME NT S TO S U PPO RT R UR AL -U RB AN T RA NS IT IO NS In tr od uc ti on Rural-urban transition zones occur in areas where high-speed roadways (e.g., in a rural environment) change or transition to low-speed roadways (e.g., in an urban environment with higher development density, higher pedestrian activity, on-street parking, etc.). When entering a lower-speed zone, particularly after a period of driving at a high speed, drivers generally underestim ate their speed and, consequently, do not reduce their speed sufficiently to comply with the lower speed limit. Infrastructure measures can help to indicate the transition from one traffic environ me nt to another and help drivers adjust to the lower speed ( 1 ). In designing and selecting transition zone measures, the goal is to have motorists traveling at the lower speed at the start of the settled area, i.e., to have the speed reduction occur in the transition zone ( 2 ). Because physical m easures are the mo st effective in reducing speed but are the mo st perilous if traversed at high speed, it may be helpful to recognize that an approach zone is required ups tream of the transition zone to warn mo torists. The transition zone and approach zone concept are shown in the figure below. De si gn Gu id e lin es Transition Zone Areas Recommended Measures Rural Area with High Speed Limit None Approach Zone Warning and psychological measures including advance signing, converging chevrons, optical speed bars, variable message signs, colored pavement, and transverse pavement markings. Transition Zone Ph ys ical measures including speed feedback signs, road narrowing, raised medians, stepped-down speed limit, roundabouts, and road diets . Settled Area with Low Speed Limit Gateway treatment at start of settled area and additional measures to manage speed within the settlement. T RA NS IT IO N Z ON E AN D A PPR OA CH Z ON E C ON CE PT S Source: Forbes ( 2 ) Based Primarily on Expert Judgmen t Based Equally on Expert Judgment and Empirical Dat a Based Primarily on Empirical Da ta HFG TRANSITION ZONES Version 2.0 9-2
Di scu ssi on A recently co mp leted NCHRP Synthesis of Highway Practice (2) exam ined hi gh-to-low speed transitions in North Am erica and overseas. While the North Am erican practice relies mainly on traffic signs, the experience from overseas is mo re varied and includes mo re extensive and aggressive m easures such as geom etric design changes, road surface treat me nts, and roadside features. The design guidance revealed in the literature review in this synthesis includes the following ( 2 ): â¢ More extensive and aggressive measures (e.g., mini roundabouts, raised speed tables) tended to produce greater reductions in speed and crash occurrence than less extensive and passive m easures (e.g., gateway signing). â¢ There needs to be a distinct relationship between an urban speed lim it and a change in the roadway character. â¢ There is not one particular measure that is appropriate for all situations. Each settlement must be assessed and treated based on its own characteristics and merits. â¢ To maintain a speed reduction downstream of the transition zone, it is necessary to provide additional measures through the village; otherwise, speeds may rebound to previous levels as soon as 820 ft from the start of the lower speed limit. Distortions in speed estim ation ma y be observed when a dr iver has to accelerate or decelerate. As a general rule, drivers usually produce a sm aller change in speed than that required. The absolute error is greater after deceleration than after acceleration. This highlights the problem s that drivers ma y face in the high-speed to low-speed transition area. Drivers make larger speed adjustm ents where the transitional situation is clearly functional (i.e., where the need to adjust the speed of their vehicle is sufficiently âobviousâ). Signing alone is not sufficient to induce appropriate speed behavior if it does not correspond to the way in which the driver perceives and categorizes the situation. Discrepancies between the structural elem ents of the road environm ent and the posted speed reinforce inappropriate driving behavior ( 1 ) . De si gn Is su es Current AASHTO policy provides a sufficient level of detail for designing roads in a high-speed environment and a low-speed environment; however, the AASHTO Green Book ( 3 ) lacks sufficient guidance on the transition zones between the two facility types. NC HRP Project 15-40 âDesign Guidance for High-Speed to Low-Speed Transition Zones for Rural Highwaysâ is currently under way. The research will develop design guidance for selecting effective geometric, streetscaping, and traffic engineering techniques for transitioning fro m high-speed to low-speed roadways, particularity rural highways entering comm unities. The final report for NCHR P Project 15-40 will identify recommended changes to the Gree n Book to address design issues related to high- to low-speed transition zones. The project has developed a table of the effectiveness of transition zone techniques based on a literature review. The co mp letion date of the project is scheduled for May 2012. Speeds chosen are lower where the height of the vertical elements (e.g., trees, large shrubs, oversized signage; 4 ) is greater than the width of the road. However, roadside elem ents need to be chosen car efully so as not to become obstacles which can have a negative effect on safety ( 1 ). Cr os s Re fe re nc es Speed Perception, Speed Choice, and Speed Control, 17-1 Signing Guidelines, 18-1 Markings Guidelines, 20-1 Ke y Re fe re nc es 1. Organization for Econo mi c Co-operation and Developm ent (OECD) and European Conference of Ministries of Transport (ECMT) (200 6). Speed Management. Paris: OECD Publishing. 2. Forbes, G. (2011). NCHRP Synthesis of Highway Practice 412: Speed Reduction Techniques for Rural High-to-Low Speed Transitions . Washington, DC: Transportation Research Board. 3. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 4. AECOM Canada Ltd., CIMA+, & Lund University (2009). International Road Engineering Safety Countermeasures and their Applications in the Canadian Context . Ottawa, ONT: Transport Can ada. 9-3 HFG TRANSITION ZONES Version 2.0